Thermal processing equipment is usually used for thermally treating semiconductor wafers in a hydrogen, argon or oxygen atmosphere, in order to, for example, form an oxide layer or to smooth the surface of a wafer. In order to achieve uniform temperature across the wafers, the equipment needs to be calibrated. For this purpose temperature sensors (or probes) are positioned below the wafer which is usually positioned horizontally in a furnace chamber. These sensors measure the local temperature of the wafer and are connected to a control system which causes the equipment to adapt locally the amount of heat provided to the wafer.
A typical method for calibrating such equipment when treating bulk silicon wafers consists of applying a rapid thermal oxidation to form about 100 Å of SiO2 with an offset of “0” for all probes (or temperature sensors). Then an oxide thickness profile across the wafer is measured revealing any thermal non-uniformities within the chamber, since the actual oxidation thickness is dependent on the temperature. Afterwards the offset values are adapted to correct for temperature non-uniformity and the process is repeated until an oxide layer with a flat or even thickness profile is formed on the wafer produced, therefore, by a uniform temperature across the wafer.
Such a direct calibration of thermal processing equipment is, however, not applicable to multilayer type substrates, for example, silicon on insulator (SOI) wafers. US patent U.S. Pat. No. 6,853,802 describes the problem that SOI-like structures have a non-homogenous structure especially at edges thereby providing local differences in the heat absorption coefficients. Thus, when treating the substrate in the thermal processing equipment, this local difference has to be taken into account, typically, by reducing the temperature applied at the edge of the wafer in comparison to the treatment applied to the bulk wafers. By doing so it becomes possible to minimize slip lines and wafer deformation, which would otherwise occur. U.S. Pat. No. 6,853,802 proposes to adapt the thermal treatment by determining the heat absorption coefficient of the structure to be treated and by adapting accordingly the power supplied to the heating lamps of the equipment. This method does, however, have the problem that the calibration is not achieved in a direct way and that in fact a correlation between the heat absorption coefficients and the equipment parameters needs to be established.
A second way of calibrating the equipment is to perform calibration experiments on actual SOI wafers to sequentially adapt the heating power so that slip lines and wafer deformation are minimized. For example, starting from equipment that has been calibrated for bulk silicon wafers, the sensor offsets, especially the outermost sensor offsets, can be adapted to take into account the different behavior of SOI and bulk wafers. To adapt the sensor offsets, a number of SOI wafers, having the same device layer and oxide layer thicknesses, are used for different values of the sensor offsets and then the slip lines and/or wafer deformation are measured using standard techniques. Finally the sensor offset providing the best results is chosen.
This method, although being a direct way of calibrating, has nevertheless the drawback that calibration requires expensive SOI wafers which will be scrapped at the end of the calibration. As thermal processing equipment suffers from continuous drift, regular recalibration is needed, and calibration using SOI wafers can become too costly to be employed on a regular basis. Finally, this second method is based on the ability to measure and identify slip lines and/or wafer deformation, which is relatively cumbersome and thus not suitable to be carried out on a regular basis.
Thus, there is a need for a calibration method for calibrating thermal processing equipment to be used for heat treatment of a multilayer substrate which easily establishes equipment calibration parameters without an excessive use of multilayer substrates.